SANTA CLARITA: Just 16 seconds passed from the time a California high school student pulled a .45 semi-automatic pistol from his backpack in an outdoor school courtyard and emptied the weapon, killing two classmates and wounding three more, before shooting himself in the head and collapsing.

Police investigating the Thursday morning shooting at Saugus High School in Santa Clarita, about 65km north of Los Angeles, said they did not yet know the motive.

The shooter’s name was not released by police, who said it was his 16th birthday. He saved his last bullet for himself, and was in grave condition at a hospital, authorities said.

Early on Friday, two girls aged 14 and 15 were being treated at Providence Holy Cross Medical Center in Mission Hills, California and were listed in good and fair condition, a hospital spokeswoman said.

At the Henry Mayo Hospital in Santa Clarita, authorities said a 14-year-old male was treated and released, and the other students died.

No names of the wounded or the dead were released early on Friday, but the two slain students were a 16-year-old girl and a 14-year-old boy.

Captain Kent Wegener of the Los Angeles County Sheriff’s Department said surveillance video footage showed the suspect opening fire from a single stationary position, shooting his victims in rapid succession before turning the gun on himself.

The scene at Saugus High School was reminiscent of other mass shootings at US schools, including Marjory Stoneman Douglas High School in Parkland, Florida, where a former student with an assault gun killed 17 people on February 14, 2018.

It was the 85th incident of gunfire at a school this year, according to Everytown, a gun control advocacy group.

Police said the accused shooter had acted alone.

Investigators descended on his family home, blocking off the street. They found no further danger there.

A next-door neighbour, registered nurse Jared Axen, said the suspect had seemed introverted and sad, possibly despondent over the loss of his father from a heart attack in December 2017.

“I would say he (the boy) was hurting and couldn’t ask for help,” Axen said of the suspect, who was a track athlete at the school, involved in Boy Scouts and liked the outdoors, going on hunting trips with his father.

There was no immediate word on where the suspect obtained the weapon.